1. Field of the Invention
This invention relates generally to the field of a manufacturing process for metal parts and more specifically, a manufacturing process for forming a base plate used in a hard disk drive.
2. Background
Hard disk drives used in computer systems are known in the art. Hard disk drives are used to store data in a computer system and may store, for example, 20 megabytes to 10 gigabytes of data. Base plates support the hard disk drive.
Conventional base plate manufacturing processes include (1) aluminum die casting, (2) press working a sheet of metal with side frames mounted on opposing sides, and (3) an extrusion method that includes press working. Each of these methods is described below.
A. Aluminum Die Casting
A base plate may be manufactured by an aluminum die casting process. Because aluminum die casting generally uses injection molding, pores are typically formed on the inside of the molded product. Pores on the inside of the product can change the properties of the aluminum material and cause corrosion or outgassing. Outgassing involves the removal of gas from a metal by heating the metal to a temperature below melting while maintaining a vacuum in the space around the metal. To compensate for these deficiencies, E-coating, a type of paint known in the art, is applied to the base plate that is produced from the aluminum die casting process.
There are numerous disadvantages related to quality, cost, and production time to using the aluminum die casting process. For example, aluminum die casting and E-coating typically result in dimensional imprecision. Therefore, the important datums need to be machined after the E-coating has been applied to the base plate. Furthermore, the quality of the base plate is affected because porosity or pin holes generally form on the surface or inside of the base plate that is produced, which affects the rigidity of the base plate. Machining of an E-coated base plate, which is required to obtain the proper dimensions, exposes the casting surface of the base plate and may cause contamination on the inside of the base plate. This reduces the reliability of the base plate. Another disadvantage to aluminum die casting relates to cost. For example, the metallic molds used in aluminum die casting are typically very expensive and have a short tool life. Additionally, there are numerous steps involved in the aluminum casting process. Each process step adds cost and increases the production cycle time. Another disadvantage is that a high yield loss typically exists due to the aluminum being melted and then injected in the cavity of the mold. E-coating typically also raises the cost of production and may have an outgassing problem.
B. Press Working with Side Frames Mounted on Each Opposite Side
Another process for manufacturing base plates relates to press working with side frames mounted on each opposite side. A base plate is press worked to form a concave portion 23 with a few holes for motor mounting 24, as illustrated in FIGS. 1 and 2. The two side frames 22 are press worked from sheet metal and are fixedly mounted on the opposite sides of the base plate.
There are several disadvantages to this conventional process. For example, fixing the two side frames 22 to the base plate is an additional assembly step that increases the cost of manufacturing. The side frames must be strictly controlled in the mounting position and the mounting strength. Another disadvantage is that relief surfaces for elements such as the disk, the actuator, the voice coil motor, the filter, and bosses or semi-pierces are not a part of this process. Instead, generally all relief surfaces are formed by a machining operation although press working may be used to form a motor mounted concave portion with a few holes. Additionally, oil and other residue that is used during the cold working operation must be removed by washing the finished base plate.
C. Extrusion Method
The extrusion method is another process used to manufacture a hard disk drive base plate. An aluminum base member is manufactured by extruding a predetermined cross-sectioned shape having a pair of frame portions on its sides 23. The extruded product is then cut to a predetermined length. The half-finished product may then be subjected to press working to form a motor mounting concave portion with a few holes as shown in U.S. Pat. No. 5,766,275 issued to Obara entitled Method of Manufacturing Hard Disk Drive Unit. This half-finished product must also be machined with a stepped hole for mounting the motor. Additional relief surfaces such as for the disk, the actuator, the voice coil motor, the filter mount and other datums are machined during machining operation.
There are several disadvantages to this process. For example, extruding material is costly because of the multiple steps involved. For instance, an aluminum billet must be formed from the aluminum ingot. The aluminum billet is then further processed. Additionally, the motor mounting “stepped hole” must be machined during a machining operation. The machining operation cycle time increases as additional relief surfaces are required.